Analytics: small fix to Smoothing configurator

- Will not crash anymore with an empty sensor tree

analytics/operators/smoothing/SmoothingConfigurator.cpp

@@ -87,7 +87,13 @@ bool SmoothingConfigurator::readUnits(SmoothingOperator& op, std::vector<Smoothi
 
     // If no inputs are specified, we pick all sensors present in the sensor tree
     if(protoInputs.empty() && inputMode==SELECTIVE) {
-        set<string> *inputNames = _queryEngine.getNavigator()->getSensors(SensorNavigator::rootKey, true);
+        set <string> *inputNames = NULL;
+        try {
+            inputNames = _queryEngine.getNavigator()->getSensors(SensorNavigator::rootKey, true);
+        } catch(const std::exception &e) {
+            LOG(error) << _operatorName << " " << op.getName() << ": Error when creating units: " << e.what();
+            return false;
+        }
         for(const auto& n : *inputNames) {
             SmoothingSensorBase ssb(n);
             ssb.setMqtt(n);